In cellular mobile radio systems it is fundamental that a mobile station with an established connection on a radio channel shall be able to maintain the established connection when moving from one cell serviced by a base station to another cell serviced by another base station. It is also highly desirable that the mobile station with an established connection on a radio channel shall be able to maintain the established connection when moving within the same cell and when the radio channel which is used is subject to increased interference. The process by which a mobile station can maintain an established connection when moving in a cellular radio system is generally called a handoff.
In general, radio communication is only possible when the desired information carrying radio signals have sufficient signal strength at the receiver and are sufficiently strong relative to the noise and interfering radio signals at the receiver. The minimum strength, of course, depends upon particular features of the system, e.g., the kind of modulation and the type of receiver. In order to insure that the established connection may continue on an intended radio channel between a mobile station and an intended base station, the handoff process includes measurements of the parameters of the radio signals at the intended base station and/or at the mobile station.
The first cellular mobile systems placed in public use were analog systems that were typically used for speech or other types of analog information. These systems include multiple radio channels for transmitting analog information between base stations and mobile stations by transmitting analog modulated radio signals. In general, the first cellular mobile radio systems had relatively large cells, and the signal measurements in the handoff process in such systems were performed by the base stations. One such system is the Nordik Mobile Telephone System NMT-450. Another known cellular radio system is the AMPS mobile radio system in the United States. A general description of mobile cellular radio systems can be found in a publication entitled "CMS 88 Cellular Mobile Telephone System", published by Ericsson Telecom AB, 1988. The rapidly increasing usage of these mobile radio systems has necessitated the development of newer more advanced digital systems that can accommodate a larger number of mobile stations using time division multiple access (TDMA) technology and code division multiple access (CDMA) technology.
In a TDMA system several mobile stations can share a single carrier signal because the signal is divided into frames. Each frame is subdivided into time slots and mobile stations are assigned to one or more time slots. Each mobile station transmits and receives short bursts of data packets during its assigned time slot. Typically the carrier signal has a bandwidth less than two megahertz.
In a CDMA-system two or more users can utilize the same time slot and/or the same carrier frequency by coding the user message so that the message of the different users can be separated from each other. In one kind of a CDMA-system the frames may be divided into different time slots as in a TDMA-system, but each of the various slots can be used by two or more subscribers. In another type of a CDMA-system, the frames are not divided into time slots but each of the carrier frequencies can be used by two or more subscribers as in a FDMA-system.
This new digital technology opens up new possibilities for smaller and improved cellular telephones, since major analog radio components can be eliminated. The relatively bulky duplex filter, which is typically used to transmit and receive separate signals is no longer needed because bursts of data are transmitted and received at different times. Moreover, requirements for filter selectively, frequency stability, voltage controlled oscillator (VCO) noise, etc. can be relaxed, thereby reducing both the cost and the size of the mobile station used in a TDMA system.
The advantage of TDMA technology for reducing the cost of a base station is even more substantial. In a TDMA system only one radio transceiver is necessary for carrying several calls simultaneously. The size of the equipment is reduced and the cost of each site with a single transmitter is also substantially reduced.
Various published standards, such as those for the GSM digital mobile system in Europe and the EIA Interim Standard (IS-54) in the United States, set forth the specifications for the transmission of a carrier signal that is modulated with digital data. Specifically, the carrier signal is divided into frames, and each frame is subdivided into time slots. For example, under the EIA Interim Standard, one frame consists of six equal time slots of approximately 6.7 ms each.
The published standards provide for data to be transmitted at a half rate or a full rate. When data is transmitted at the full rate, the traffic channel in which that data is transmitted utilizes two of the time slots in a frame. Thus, under the EIA Interim Standard, each frame provides three full rate traffic channels. Typically, the six time slots are numbered, 1,2,3,1,2,3, so that one channel uses the first and fourth time slots in a frame, another channel uses the second and fifth time slots, and the third channel uses the third and sixth time slots. Each packet of data that is transmitted over a channel in a frame is divided between the two time slots for the channel. Specifically, the data is interleaved between the two time slots to reduce susceptibility to noise and interference. By interleaving the data between two spaced time slots and the use of appropriate error correction, data bits which are lost in one time slot due to noise or interference can be reconstructed from data received in the other time slot of the channel.
In the alternative half rate mode of transmission, each channel utilizes only one time slot in each frame. Thus, a single frame can accommodate six traffic channels under the EIA Standard, thereby doubling the call-handling capacity of the system. However, since data packets are not interleaved between plural time slots in the half rate mode of transmission, there is a greater susceptibility to the loss of data due to noise or interference in this mode. As a result, cells which are designed for full rate transmissions cannot handle half rate transmissions with appropriate transmission quality over the entire cell area.
It is desirable to utilize the advantageous features of both of these transmission modes to provide the increased call-handling capabilities afforded by the half rate mode of communication while at the same time ensuring the high quality data transmission provided in the full rate mode to enable communication with appropriate signal quality over the entire cell area.